Ultrafast, autonomous self-healable iontronic skin exhibiting piezo-ionic dynamics

The self-healing properties and ionic sensing capabilities of the human skin offer inspiring groundwork for the designs of stretchable iontronic skins. However, from electronic to ionic mechanosensitive skins, simultaneously achieving autonomously superior self-healing properties, superior elasticity, and effective control of ion dynamics in a homogeneous system is rarely feasible. Here, we report a Cl-functionalized iontronic pressure sensitive material (CLiPS), designed via the introduction of Cl-functionalized groups into a polyurethane matrix, which realizes an ultrafast, autonomous self-healing speed (4.3 µm/min), high self-healing efficiency (91% within 60 min), and mechanosensitive piezo-ionic dynamics. This strategy promotes both an excellent elastic recovery (100%) and effective control of ion dynamics because the Cl groups trap the ions in the system via ion-dipole interactions, resulting in excellent pressure sensitivity (7.36 kPa−1) for tactile sensors. The skin-like sensor responds to pressure variations, demonstrating its potential for touch modulation in future wearable electronics and human–machine interfaces.


Supplementary Note 1: Synthesis, morphological characterization of CLPU film, and the optimization approach of CLiPS with various ionic liquid concentration (10-40 wt%).
The fabrication of the self-healing CLiPS primarily involves three steps (Supplementary Fig.   1 and Fig. 1c, Main manuscript) (i) the synthesis of poly(epichlorohydrin-co-tetrahydrofuran) diol (PET), (ii) the synthesis of Cl functionalized polyurethane (CLPU), and (iii) the preparation of a CLiPS film. Supplementary Fig. 1 presents the synthesis process of CLPU under optimized conditions. The PET was synthesized via cationic ring opening polymerization 1 , as mentioned in the Method section (Main manuscript), using epichlorohydrin (ECH) and tetrahydrofuran (THF) to form the soft segment of the polyurethane (PU) structure. The ECH component contains the Cl functional groups (Cl groups), which is the key component in our PU structure for piezo-ionic dynamics.
Theoretically, all halogen groups can be utilized as trap sites to established ion trap and release phenomenon via ion-dipole interactions. Nevertheless, we introduced the Cl group among halogen ones owing to higher dipole moment in this work. In general, dipole moment is a product of charges and the distance between the charges in a molecule. The Cl group has been reported to generate the highest dipole moment, due to the longer distance and high partial charges between the C and Cl atoms 2,3 . Although the F group possesses superior charge (highest electronegativity), the distance between the C and F atoms are the lowest, thereby creating a lower dipole moment in the F group. In contast, the Br group possesses comparably longer distance between atoms due to the larger size of the Br atom. However, because it possesses a very small charge, it tends to exhibit a lower dipole moment. A lower dipole moment would correspondingly weaken the trapping effect on ions. Therefore, in this work, the Cl group was chosen because it exhibits the optimum conditions for higher dipole moment for the establishment of strong trapping effect on ions.
Through one-step polymerization method, CLPU samples were synthesized using PET, dynamic disulfide bonds (BHPDS), and isophorone diisocyanate (IPDI) at a molar ratio of PET:BHPDS:IPDI = 8:1:9 using dibutylin dilaurate as catalyst. The IPDI and the BHPDS constitute the hard segment, and the ECH constitutes the soft segment of the polyurethane, respectively. The hard segment can be defined as a chain segment formed by the reaction of diisocyanate with chain extender on the polyurethane main chain. Likewise, the soft segment can be defined as a chain segment consisting of oligomeric diols with glass transition temperature (Tg) below room temperature 4 . To establish the optimum content of Cl groups, various CLPU samples were synthesized by optimizing the ratio of the ECH concentration in the soft segment with a molar ratio of E3 (ECH 3:7 THF), E4 (ECH 4:6 THF), E5 (ECH 5:5 THF), E6 (ECH 6:4 THF), and E7 (ECH 7:3 THF), where E3 and E7 have the lowest and highest Cl groups composition, respectively.
We postulated that, more Cl groups composition would reduce the self-healing speed due to their intrinsic toughness properties, whereas increased Cl groups are necessary for better ion trapping effects for effective piezo-ionic dynamics. Hence, with a stepwise increase in the content of Cl groups, the self-healing speeds of E3, E4, E5, E6 and E7 were analyzed. Here, the self-healing speed (µm/min) can be defined as the speed taken by the damaged sample to attain maximum selfhealing efficiency with or without external stimuli. Generally, the self-healing speed is calculated based on the ratio of [notch size] (µm) / self-healing time (min) 5,6 . However, in this work, the sample was cut completely with the knife piercing through to the lower part, hence, the notch size was taken as the thickness of the sample. Also, the self-healing efficiency was determined based on the ratio of (the stress of healed sample / the stress of original sample) 7 x 100%. The formula are as follows: Using the appropriate equivalent circuit models built in NOVA software (Metrohm Autolab), all the impedance spectra were fitted, and the bulk resistance (Rb) of the devices was evaluated.
where D is the diffusion coefficient, ρ is the charge density,  -1 is the Debye length.
where ∈r is dielectric constant of electrolyte, ∈0 permittivity of free space, kB Boltzmann's constant, T temperature, NA Avogadro constant, I the ionic strength, ci ion concentration and zi is ionic charge.
From these expressions, the Debye length is inversely proportional to the ionic strength. Therefore, the Debye length decreases with higher number of free ion density. Herein, our system function similar to the RC (bulk resistance; R = d/σA, bulk capacitance; C = ∈A/d) circuit where the time scale at which the ion dynamics become diffusive is known as the charge relaxation time (τ = ∈/σ = RC) and the crossover frequency of the imaginary and the real impedance in the Bode plot is referred to as the charge relaxation frequency (τ -1 ) 17 . It is therefore expected that the impedance from R and C decreases with an increase in pressure. We note that the CLiPS-based e-skin device exhibited a distinguishable decreasing impedance plot owing to the released of more mobile ions under increasing pressure ( Supplementary Fig. 11a). In contrast, the CLPU@E0-IL-based device exhibits no significant change in impedance plot. This is because the ions are not initially trapped, and it is difficult to expect pressure-mediated ion pumping for the release of ions under pressure (Supplementary Fig. 11b). In addition, as the concentration of free ion number density increases, the electrode polarization would shift toward a higher frequency regime 16 . The frequency dependence of tan δ (Ɛ''/Ɛ') can be utilized to explore the free ion number density and diffusivity by analyzing the shift in relaxation peaks corresponding to the angular frequency (ꞷmax), at which the full development of electrode polarization takes place 13,18 . With the increase in free mobile ion number density and diffusivity under increasing pressure, τ -1 in the CLiPS shifts toward higher frequencies owing to the faster ionic atmosphere relaxation (Fig. 4c, Main manuscript). However, τ -1 in the CLPU@E0-IL (Fig. 4d, Main manuscript) exhibits no evident shift because ions are not released with the increase in pressure, as the majority of the ion pairs already exist within the free volume of the matrix 8 . Therefore, the CLiPS-based device exhibits shorter Debye length (high average free ion density in the diffusion layer), which results in the shift of τ -1 towards higher frequencies (shorter times), signifying lower relaxation time at higher pressures (Fig. 4e, Main manuscript). From the maximum peak in tan δ, the relaxation time can be expressed as 1/ꞷmax.

Supplementary Note 3: Pressure response of CLiPS films under different applied bias voltages and bias frequencies
The pressure responses of different CLiPS (10-40 wt%)-based mechanosensitive pressure sensors as depicted in Supplementary Fig. 13 and Supplementary Fig. 14